1. Technical Field
Aspects of this document relate generally to telecommunication systems, such as satellite, wired, and wireless systems.
2. Background Art
A growing number of satellite communication systems use time division multiple access (“TDMA”) protocol for communications between earth stations or sites. In order to effectively control these networks, a single node, often referred to as a central site, hub or master terminal, is responsible for administering and policing the network. It is through this master terminal that new remote sites can be added, removed, or have their operating parameters, such as their bandwidth allocation, modified. This master station is also responsible for identifying remotes that have temporarily gone missing due to unforeseen circumstances, such as a power failure, and for successfully bringing them back into the network.
Because the leadership and control of the network rests solely upon the hub or master station, they present a single point of failure. Thus, a backup master station or redundant hub is often required to insure that the network will keep working if the prime unit suffers a failure. This additional equipment adds complexity and cost to the network, but is seen as a necessary evil.
Also with TDMA networks, a communication medium is shared amongst several users by dividing the medium into timeslots and then allocating those timeslots to the various users. For satellite communications, this medium typically consists of a single carrier frequency, or a set of multiple carrier frequencies. In traditional TDMA networks, the remote sites run in a “slave or dumb” operating mode, and thus do not have the ability to allocate bandwidth on their own. These sites rely on the hub or master station to coordinate their timeslot and/or frequency allocations. This reliance on a central controller adds latency to the network because a double satellite hop is required before bandwidth can be allocated.
The remote sites transmit their requests to the central site and then the central site transmits back the allocations to the remote sites. In some cases, such as an Aloha system, collisions are an unavoidable part of the system architecture which adds additional latency as the remote sites must transmit their bursts, wait to see if a collision has occurred, and if so, fallback and retransmit the burst.
In TMDA networks, users effectively share the communication medium by transmitting bursts of data at the prescribed times associated with their timeslots. Accurate timing is required in order to keep bursts from colliding or overlapping, which would cause a loss of data and necessitate retransmission. This is especially true of satellite network where the inherent movement of the satellite causes a shift in timing at every site in the network.
In traditional TDMA networks, accurate timing is typically achieved by either:                a) “Slaving” the remotes sites to a central hub or master station where a high stability clock is installed or an external reference source is available; or        b) Providing a high stability reference such as GPS at all of the sites. This can be cost prohibitive, especially in the case of large network.        
With either traditional solution, the remotes will lock their internal clock to the provided reference (master station or local GPS), to prevent clock slippage relative to one another. Periodic updates are then sent out to adjust for the satellite movement. These adjustments are typically sent by the hub or one of the remotes that has been designated as the timing leader. Appropriate backup capabilities must be provided to insure that the network timing is preserved if the hub or timing leader is shut down.